It is known that certain cations and anions, have corrosion inhibiting properties and that compounds containing them can be included in protective films and coatings that are intended to provide adhesion and corrosion inhibiting properties to metallic surfaces and structures. Typical examples include cations of calcium, magnesium, strontium, barium, manganese, zinc, cerium and other rare earth elements as well as anions such as silicate, borate, molybdate, nitrophthalate, phosphate, hydrogen phosphate, phosphite, phosphonates and phosphonocarboxylates. Classic inhibitors based on lead compounds and cations and compounds of some other heavy metals such as chromium e.g. chromate and zinc are however of less interest these days for environmental and health and safety reasons.
The inhibitive compounds may be in the form of sparingly water-soluble salts and can for example be prepared by a process of particle growth and precipitation in the presence of the required cations and anions under suitable conditions. The inhibitive compounds may also be in the form of particles of inorganic oxides such as silica, silicates, alumina and aluminosilicates comprising additional inhibitive cations and anions. These inhibitive compounds can for example be prepared by a process of precipitation or gelation of the oxide in the presence of the required cations and anions under suitable conditions.
Inhibitive compounds based on inorganic oxides can alternatively be made through a process of ion-exchange, in which surface protons and hydroxyl groups of the pre-formed oxide are replaced by contacting the oxide with a solution containing the required inhibitive cations and anions, again under suitable conditions. In either case, the inorganic oxides involved are often characterized by having certain surface areas and porosities with corrosion inhibiting ions attached to the internal and external surface of the particles, producing surface modified inorganic particles, although ions may be found through the bulk of the particles as well, depending on the method of preparation.
Of course, from the above description, combinations of inhibitive compounds based on sparingly soluble salts and those based on inorganic oxides may be prepared simultaneously in various ways according to the composition of the solution or slurry from which the inhibitive compounds are to be prepared and the processing route, allowing for a great variety in properties displayed by the resulting inhibitive compound.
In many cases, the films and coatings employed in anti-corrosion have a certain permeability to water and it is believed that the mechanism of corrosion inhibition involves gradual dissolution of the compounds in water, releasing ions as the active inhibitors. For such systems to be effective over a long period, the solubility of the compound is particularly important. If the compound is too soluble, blistering of the coating may occur and the compound will be rapidly depleted; if it is insufficiently soluble the compound will be ineffective. Whether the inhibitive compound is a sparingly soluble salt, or based on an inorganic oxide or is some combination of the two, the typical solubility of such compounds suitable for use in films and coatings results in inhibitive ion concentrations in aqueous media of around 10−5M to 10−2M.
For inhibitive compounds based on inorganic oxides, the inorganic oxide may itself have a certain solubility with respect to the provision of inhibitive substances, according to the nature of the environment in which the corrosion inhibiting particles are used e.g. in the case of silica, silicic acid has a background solubility of about 10−3M with the concentration of silicate being pH dependent and having a value of 10−2M for example at a pH of about 10.5. It is however sometimes believed that these types of corrosion inhibiting particles can act to release inhibitive cations and anions into solution by ion exchange with aggressive ions existing in that environment as an additional or alternative mechanism of action to one based on dissolution. The rate of release of the corrosion inhibiting ions would then be influenced by the permeability of the film or coating to the exchanging ions in addition to or rather than dissolution of inhibitive ions into the permeating aqueous environment. Corrosion inhibiting ions would in that case be released to a greater extent from the inorganic oxide in those areas where the desired barrier properties of the coating were weakest, leading thereby to improved performance properties.
The inhibitive compounds referred to above are usually made available in the form of dry powders, making use of washing, drying and milling operations as required as additional processing steps and average particle sizes of the powders usually exceed about 1 to 2 microns making them most suitable for films and coatings that exceed a few microns in thickness. For suitability of incorporation into a wide variety of film and coating systems as well as suitability with respect to corrosion resistance, besides the criteria of solubility, the pH of an aqueous slurry of such inhibitive compounds will in most cases typically fall within the range of 4 to 10.5 although lower or higher values can be suitable depending on the actual chemistry of the coating or film in question and the nature of the metallic substrate. For example, many surface pre-treatments can be quite acidic and display a pH in the range of 1 to 4.
Making anti-corrosive particles available in dispersion form would render the operation of incorporating the particles into the coating more convenient and would avoid generation of dust. However, like the dry powder, known anti-corrosive particle dispersions contain pigment particles that are relatively large in size and greater than about 1 to 2 microns. Such dispersions are not suitable for use in filming and coating applications that require small particle sizes, such as chromium-free surface treatments or thin film primers where the film thickness may be less than a few microns going down to the sub-micron region. It is additionally believed that both the particle size and state of dispersion of the inhibitive pigment may influence the availability and mobility of inhibitive ions derived from the inhibitive compound within the film or coating under environmental exposure. Small particle pigments may therefore provide potential benefits in anti-corrosive treatment films and coatings regardless of the thickness of the film applied.
The general principles covering the operations of milling, dispersion and dispersions of particles such as inorganic and organic coloring pigments or fillers in liquids and surface coatings, as well as the properties of such suspensions are of course well known. For example, a comprehensive review including the role of surface active, wetting and dispersion agents as a means of producing and stabilizing pigment dispersions is given in “Dispersion of Powders in Liquids with Special Reference to Pigments” 3rd Edition, Edited by G. D. Parfitt, Applied Science Publishers, 1981. Further details on the fundamentals and preparation of the colloidal state can be found in “Foundations of Colloid Science” by R. J. Hunter, Vol 1-2, Academic Press, 1986.
Needless to say, a number of patents discuss specific aspects of the dispersion of particles and pigments and the properties of particulate and pigment suspensions with respect to surface active, wetting and dispersion agents and other additives designed to control some feature of the dispersion. U.S. Pat. No. 4,186,028 for example concerns the use of phosphonocarboxylic acids as dispersants to lower the viscosity and reduce settling of highly concentrated aqueous suspensions of pigments and fillers such as titanium dioxide, iron oxides, zinc oxides, chromium oxides, talcs, calcium carbonate, barium sulphate and quartz at a level of phosphonocarboxylic acid ranging from 0.01% to 1% based on the pigment solids. The preferred pH range of the suspensions was 6 to 10.
It should be noted that many surface active, wetting and dispersion agents and other additives can display a certain compatibility or even solubility in water and as such can detract from the performance properties of coatings and film forming layers intended to provide anti-corrosive and/or adhesion promoting properties. The type and amount of such substances may therefore require careful selection.
In overview, for convenience in use, for suitability in thin films and for performance gains, there is therefore interest and a need in the industry for anti-corrosive small diameter particle dispersions that may be utilized in a variety of treatment films, primers and coatings. Ideally, these dispersions would be free of compounds of lead, chromium and some other heavy metals such as zinc. The present work is therefore concerned with the question of producing stable small particle dispersions of inhibitive compounds suitable for use in aqueous and non-aqueous protective films, surface treatments, primers, coatings, adhesives and sealants that are intended to provide adhesion and corrosion inhibiting properties to metallic surfaces and structures.